A flying mammal of the order Chiroptera. There are two sub-orders of bats, the Megachiroptera or flying foxes and the Microchiroptera which are smaller insectivorous bats. The wings of the bat evolved from the forelimbs; the finger bones are greatly elongated and joined to the forearm by a membrane, while the thumb is shaped into a claw which is used for catching insects or hanging on to surfaces. Bats have large ears, which helps them to locate objects by sonar - they can catch two flies in a second or detect and avoid a wire of diameter 0.1 mm. The head of a bat is usually rodentlike or foxlike. Bats vary greatly in size: the largest species, Pteropus vampyrus, has a wing span of 1.5 m, but the smallest bat, Craeseonycteris thonglongyai, has a wing span of only 15 cm.

Bats are found worldwide and usually live in colonies. They hunt at night and roost during the day. Some species roost in isolated locations such as caves, while others like the heat and dryness of external roosts such as tree trunks. Many species hibernate during the winter, and have to migrate between summer roosts and winter places of hibernation (such as a cave where the temperature is cool but not below freezing). Bats often form separate colonies for the different sexes, or clusters of each sex within a mixed-sex colony. Pregnant females go to separate "nursery roosts", which are sometimes chosen because they are hotter than average. Small bats suckle their young for 5-6 weeks, larger flying foxes for 5 months.

The commonest species of bats eat flying insects. The flying foxes eat fruit, and some rarer species of bat eat flowers, fish or small animals, while vampire bats live on the blood of larger birds and mammals. Bats are long-lived (some have lived for over 20 years) perhaps because roosting in secure places protects them from predators and their sonar makes them good at hunting and detecting threats.

The bone structure of a batwing, which still strongly shows signs of its origins as a more normal weight-bearing limb, poses an interesting problem:

If bats did, as is theorized by evolution, develop their unique characteristics as a species over a long period of time, there would have to have been some intermediate stages. You couldn't go from foreleg to fully functional wing in one generation. But the idea of survival of the fittest would hold that those new forms had to have some sort of advantage to survive and reproduce.

So what could those in-between proto-bats have done with their new limbs that weren't quite legs and weren't quite wings?

Bats are incredibly common throughout the world. They probably outnumber all other animals in total numbers. There are over 850 known species of bats, and they are second only to rodents in number of species. Every continent in the world has bats, with the exception of Antarctica, and some of the species have existed in their present form for over 50 million years. The vast majority of bats live in tropical regions, with only 44 species known to exist in North America. Bats belong to the mammalian order Chiroptera, meaning "hand-wing".

Adult bats in North America range in size from less than a tenth of an ounce and having a wing span of 4 inches to weighing over 2.2 pounds and having a wing span of 6 feet. Most bats eat insects, although some eat fruit, vegetation, and even rarely frogs, small rodents and blood. Bats are incredibly important in controlling insect populations. One bat can eat an amazing 1000 to 3000 mosquitos in a single night. Because of this, many people are now putting bat nesting boxes in their yards, or planting bat gardens, with plants designed to attract bats. Bat watching is becoming a popular form of eco-tourism. Wildlife viewers from around the world visit Bracken Cave in Texas to watch over 20 million free-tailed bats rise in huge columns at dusk in pursuit of over 250,000 pounds of insects each night. One cave in Texas, the Eckert James River Bat Cave houses an incredible eight million bats each summer. This particular cave is a maternity colony where female nest before returning to Mexico for the winter.

One of the Cheiroptera, an order of flying mammals, in which the wings are formed by a membrane stretched between the elongated fingers, legs, and tail. The common bats are small and insectivorous. See Cheiroptera and Vampire.